1. Field of the Invention
This invention relates to an image coding system wherein coding of still image data is controlled so that the amount of generated codes after coding may be substantially fixed irrespective of the image, and more particularly to an image coding apparatus suitable for use with an image processing apparatus wherein a plurality of originals are read in from an image inputting apparatus and stored onto an image recording medium and then they are read out by a designated number of times in a designated order and outputted from an image outputting apparatus.
2. Description of the Related Art
One of functions of an image processing apparatus such as a digital copying machine is an electronic sorter function of reading out data of a plurality of originals read in from an image inputting apparatus and stored on an image storage medium by a designated number of times in a designated order and outputting them from an image outputting apparatus. In order to store a large number of images onto an image recording medium having a limited capacity in an image processing apparatus (hereinafter referred to as electronic sorter) having such an electronic sorter function, coding of inputted image data is performed.
FIG. 9 shows an example of construction of an electronic sorter. Referring to FIG. 9, when a plurality of original images are inputted one by one from an image inputting apparatus 1, the inputted images are coded one by one by an image coding apparatus 2. Then, the coded image data are temporarily stored into a buffer memory 3 and finally recorded onto an image recording medium 4. After the plurality of images inputted from the image inputting apparatus 1 are all recorded onto the image recording medium 4 in this manner, outputting of the images is started. The images recorded on the image recording medium 4 are read out and decoded in a designated order by an image decoding apparatus 5 by way of the buffer memory 3 and outputted from an image outputting apparatus 6.
By the way, as a representative one of high efficiency coding systems for still natural image data, for example, the ADCT coding system which is a combination of orthogonal transform and variable length coding is known. It has been decided that the ADCT coding system is adopted as the international standards for the color still picture coding system (Magazine of the Image Electronic Engineering Society of Japan, Vol. 18, No. 6, pp. 398-409), and it can be applied also as a coding system for an electronic sorter. An outline of the ADCT coding system will be described below.
In the ADCT coding system, an input image is normally divided into blocks of 8.times.8 picture elements, and two-dimensional DCT transform (discrete cosine transform) is performed for each block to produce 8.times.8 DCT coefficients. By converting the actual space of an image into a frequency space by such DCT transform, information included in the image can be concentrated upon DCT coefficients of low frequency components having a good visual characteristic. Each of the DCT coefficients is quantized with a quantization step width determined taking the visual characteristic into consideration, and the thus quantized DCT coefficients are scanned in order from the lowest frequency component coefficient to re-arrange them into one-dimensional data, whereafter coding such as Huffman coding is performed for the one-dimensional data. The ADCT coding system is a kind of variable length coding system, and when the coding parameter is fixed, it generates a different amount of codes depending upon the image.
When the ADCT system is applied to an electronic sorter, the amount of codes of coded images must be controlled to be equal to or lower than the capacity of the buffer memory 3. To this end, several methods of controlling the code amounts for each image have been proposed.
For example, Japanese Patent Laid-Open No. Hei 4-48874 proposes a method wherein an aimed code amount and an actually generated code amount are compared with each other for each image and the coding parameter is adjusted in accordance with a result of the comparison to repeat coding so that the generated code amount may be equal to or lower than the aimed code amount to control the code amount. Japanese Patent Laid-Open No. Hei 4-220878 proposes another method wherein a generated code amount is measured using a plurality of different coding parameters for each picture element to find out a relationship between the quantization parameter and the generated code amount and a quantization parameter with which an aimed code amount may be generated is estimated from the relationship, and then coding is performed using the thus estimated quantization parameter to control the code amount.
While the two methods can control the generated code amount to be substantially equal to an aimed code amount, a plurality of coding processes are required to effect compression coding of an image, and there is a problem in that the processing speed is low.